In a fuel cell, a unit cell is composed of a fuel cell including a membrane electrode assembly (MEA) having an electrolyte layer of a solid polymer membrane or the like, diffusion layers of carbon cloth, carbon paper, or the like sandwiching the electrolyte layer, and separators sandwiching the membrane electrode assembly. In such a unit fuel cell, hydrogen gas, serving as an anode gas, is supplied to a hydrogen gas passage groove of a separator on a negative electrode side while air (oxygen gas), serving as a cathode gas, is supplied to an oxygen gas passage groove of a separator on a positive electrode side. The supplied hydrogen gas and oxygen gas diffuse into the respective diffusion layers on the negative electrode side and positive electrode side. The hydrogen gas reaching the diffusion layer of the negative electrode further contacts a catalyst layer applied on a solid polymer electrolyte membrane, and dissociates into protons and electrons. The dissociated protons permeate through the solid polymer membrane, and move toward the positive electrode to react with oxygen at the positive electrode to thereby produce water and electricity. Generally, a plurality of unit cells described above having such a power generation mechanism are used, and stacked with separators interposed therebetween to form a fuel cell as a whole as serially connected cell modules or cell stacks.
FIG. 13 is a cross sectional view showing a fuel cell 500 of the related art. FIG. 12 is a plan view showing a separator 20 stacked in the fuel cell 500 shown in FIG. 13. The fuel cell 500 has a unit fuel cell 40 including an MEA 30, and a first separator 10 and a second separator 20 sandwiching the MEA 30. Note that the first separator 10 and the second separator 20 are bonded with an adhesive 22, and that the MEA 30 is sandwiched therebetween. The adhesive 22 also functions as a sealing agent for sealing a fluid (gas, coolant) supplied to an FC cell. The thus formed unit fuel cells 40 are further joined with the adhesive 22, so that the unit fuel cells 40 are affixed to each other in a physically solid manner to form a cell module. The cell module has a manifold 80 therein for passage of a gas or a refrigerant.
The separator 20 is provided with a separator outer peripheral weir 75 on the outermost peripheral surface thereof, and at a section interior thereto a manifold outer peripheral weir 76 is provided on an outer peripheral surface of the manifold 80 surrounding the same. At a further inner section, the separator includes a separator inner weir 77.
The separator 20 has a recessed groove functioning as an adhesive-holding section 24 for holding the adhesive. Referring to FIG. 13, the recessed groove includes an outer peripheral recessed groove 72 formed between side surface walls; i.e. the separator outer peripheral weir 75 and the manifold outer peripheral weir 76 on the outer peripheral side of the separator, and an inner peripheral recessed groove 74 formed between side surface walls; i.e. the manifold inner peripheral weir 76 on the inner peripheral side of the separator and the separator inner peripheral weir 77.
The method of bonding the fuel cells 40 will be described with reference to FIG. 14. In the first and second separators 10 and 20, on opposing surfaces of the fuel cells in a direction where the layers are joined and stacked, a recessed groove 24 is provided with a side surface 29 positioned at right angles to a bottom surface 23. The adhesive 22 is applied in the right-angled recessed groove 24 of the second separator 20. After application of the adhesive 22, the first and second separators 10 and 20 are positioned so that the recessed grooves 24 of the first and second separators 10 and 20 face each other, thereby forming the adhesive-holding section 24 with its side surface 29 positioned perpendicular to the bottom surface 23. With the adhesive-holding section 24 thus formed holding the adhesive 22, the first and second separators 10 and 20 are bonded, thereby forming a module of the fuel cells 40.
The following references are known as disclosing the bonded separators. Patent document 1 (Japanese Patent Laid-Open Publication No. 2002-260691) described below discloses a separator for fuel cells in which a recessed pit having the side surface 29 disposed at right angles to the bottom surface 23 is provided around the manifold and a cooling water passage groove between the first and second separators, and a gas-impermeable adhesive is injected into the pit to bond the two plates, thereby suppressing gas permeation. Patent document 2 (Japanese Patent Laid-Open Publication No. 2002-367631) discloses a sealing structure having a pit for a material bubble between the bonded plates. Below-described patent document 3 (Japanese Patent Laid-Open Publication No. 2000-48832) discloses a fuel cell separator having a weir for preventing protrusion of the adhesive. Patent document 4 (Japanese Patent Laid-Open Publication No. 2001-319666) and patent document 5 (Japanese Patent Laid-Open Publication No. 2001-319676) described below disclose a separator sealed by a liquid seal applied in a groove provided surrounding a communicating hole.
However, the conventional recessed pit having the side surface 29 positioned at right angles to the bottom surface 23 may allow a gas, such as air, into an interface between the surface of the pit and the adhesive (sealing material) when the separators are assembled and bonded, whereby the adhesive may fail to completely fill in the recessed pit and thereby create a gap. Such a gap may lead to insufficient adhesion between the separators.
The reason why a gas is introduced during the step of bonding the separators will next be discussed. The adhesive 22 is applied to the recessed pit having the side surface 29 positioned at right angles to the bottom surface 23 in the second separator 20. Such an application of the adhesive 22 may allow a gas in between the surface of the recessed pit and the adhesive 22, and the gas introduction results in a gas bubble 28.
When in such a state the first separator 10 is provided on the second separator 20 to bond the two and form a module (FIG. 14), the first and second separators 10 and 20 are bonded with the gas bubble 28 remaining in the recess (FIG. 15). By thus bonding the separators while the gas bubble remains may lead to the following problems caused by the gas bubble 28.
(1) When a thermosetting adhesive is used for the adhesive 22, a step of thermally curing the adhesive is required. In such a thermosetting step, heat is transmitted not only to the adhesive but also to the gas bubble 28, which expands due to heat (FIG. 16). The expanded gas bubble 28 deteriorates adhesiveness of the adhesive 22, and may also partially break the adhesive and lead to gas leakage. In such a state, adhesion between the separators deteriorates, and the gas leaks through the adhesive (sealing agent) with lowered function.
(2) Fuel cells, including a solid polymer fuel cell operable at a relatively low temperature, are usually operated at a temperature of 70° C. to 80° C. At such a temperature higher than ordinary temperature, the gas bubble is also thermally expanded (FIG. 16). Such expansion of the gas bubble 28 may cause problems similar to those discussed in item (1).
The first separator 10 and the second separator 20 forming a unit fuel cell are affixed together with the adhesive 22, and the MEA 30 is sandwiched therebetween. The first and second separators have various stepped sections at the adhesion section. More specifically, the MEA 30 has a triple layered structure with the diffusion layer formed on either side of the electrolyte layer, and only the electrolyte layer extends outward, where it is sandwiched and fixed by the first and second separators 10 and 20. At least one of the first and second separators 10 and 20 forms a recessed section corresponding to the section sandwiching the electrolyte layer (see patent document 2).
A gas bubble is formed also at an end of the recessed section; i.e. a corner of the bottom surface and the side surface of the stepped section. Further, a hydrogen gas (fuel gas) or an oxidized gas (air) is supplied to a space between the MEA 30 and the first separator 10 or the second separator 20. Therefore, a channel for such a gas and the space must be brought into communication, and a sealing plate is used to prevent the adhesive from penetrating the communicating channel. A stepped section is formed in one of the first and second separators 10 and 20 corresponding to the end of the sealing plate, and a gas bubble is formed also at the corner of the bottom surface and the side surface of this stepped section.
When the gas in the gas bubble is heated and expanded during the bonding step, a leakage path between an internal space of the cell and the manifold functioning as the passage for the refrigerant may be formed.